1. Field of the Invention
The present invention relates to a dust-proof film (hereinafter referred to as "pellicle") which is used for preventing a foreign substance such as dust from adhering to a photomask, a reticle or the like (hereinafter referred to as "mask") used at the lithographing step in the process of fabricating a semiconductor device such as an IC or LSI.
2. Description of the Related Art
At the lithographing step, a mask comprising a glass sheet and a circuit pattern of a vacuum deposition film of chromium or the like formed on the surface of the glass sheet is used, and the circuit pattern is transferred to a resist-coated silicon wafer. At this step, if light exposure is carried out when a foreign substance such as dust is adhered to the circuit pattern on the mask, the foreign substance is transferred to the wafer and a defective product is formed. Especially when light exposure is carried out by using a stepper, the risk that all of the chips formed on a wafer will be defective is increased, and adhesion of a foreign substance to a circuit pattern of a mask or the like is a serious problem. Therefore, as a means of eliminating this problem, a pellicle has recently been developed and used.
In general, the pellicle comprises a transparent film or the like spread on one side face of a pellicle frame composed of aluminum, a double-adhesive tape is applied to the other side face of the pellicle frame, and the pellicle is attached to a mask. If this pellicle is used, the intrusion of a foreign substance from the outside can be prevented, and even if a foreign substance adheres to the film, the foreign substance is transferred in the fuzzy state during light exposure and no particular problem arises. Nevertheless, where a foreign substance adheres to the inner face of the film or pellicle frame, if vibration or shock is generated, for example, during an exchange of the mask on the light exposure apparatus, the foreign substance may drop onto the mask, resulting in the formation of defective products.
Japanese Unexamined Patent Publication (Kokai) No. 60-502121 discloses a method in which a protecting cover is applied to the bonding face on the mask side of the pellicle frame. But, when the protecting cover is peeled at the time of using the pellicle, the pressure of the interior is reduced and sometimes a foreign substance is engulfed therein together with air. Accordingly, with the increase of the integration degree in a semiconductor device and the reduction of the allowable size of a foreign substance, it is feared that even a fine foreign substance having a size of 1 to 2 .mu.m or less will cause a problem.
The inventors found that, when a clean pellicle is attached to a mask after confirmation of the absence of a foreign substance adhering to the inner side of the film or pellicle, sometimes a foreign substance, not observed before the attachment, adheres to the mask. To clarify the cause of this undesirable phenomenon, the inventors carefully examined the interior of a pellicle, and as a result, found that fine convexities and concavities, as shown in FIG. 2, are formed on an alumetized aluminum frame customarily used for the pellicle, and that the section of a double-adhesive tape customarily used for bonding the pellicle to a mask is porous, as shown in FIG. 3. Moreover, it was found that a foreign substance contained in such a dent causes a generation of dust upon vibration or shock, and it is very difficult to detect or remove a foreign substance of this type. Accordingly, it was concluded that, even if a pellicle having no foreign substance on the surface is obtained and an intrusion of a foreign substance from the outside is completely prevented, a generation of dust cannot be prevented in the conventional structure.